The present invention relates to a parking lamp operating system which reduces the electrical load on the batteries of a vehicle, such as a heavy duty truck, during periods when the engine of the vehicle is turned off but the parking lamps of the vehicle must remain on and more particularly, to a system which modulates a signal from the battery to provide a waveform output to control a switching means, such as a power transistor, disposed between the battery and the parking lamps, and thereby provided a pulse-width-modulated output to the lamps, lowering the effective voltage thereto to conserve power, while providing sufficient light output to be observed.